OBRABOTKAMETALLOV Vol. 26 No. 3 2024 technology welding and after additional heat treatment. Heat treatment modes for welded pipe specimens were: normalization at 950–1,000 °C, tempering at 780–800 °C. Heat treatment was carried out on an induction installation MGZ-102 with a current frequency of 2,500 Hz, in a ring split single-turn inductor with an internal diameter of 40 mm and a width of 30 mm. The tempering modes (temperature 500–600 °C) were selected experimentally by changing the current in the range of 25–30 A and adjusting the generator excitation voltage 180–200 V, while the power was 5–6 kW. And in the normalization mode (850–1,000 °C), the current was in the range of 50–60A, the generator excitation voltage was 350–370 V, and the power was 17.5–22.2 kW. The effect of welding parameters on the thermal cycles of the weld, the microstructure and mechanical properties of the joints was studied by changing the upsetting pressure, upsetting allowance and upsetting current time, under the condition that the main parameters were constant. The performance and failure behavior of various welded pipes were estimated by conducting mechanical tensile tests on an Instron electromechanical testing machine with a lifting capacity of 1,000 kN. Hardness according to the GOST requirements for heat-resistant steels was determined on a Brinell hardness tester ITB-3000-IIIAZhP. The welded specimens were subjected to the flattening and bend tests, which are a quality control tests to evaluate the ductility and integrity of the butt weld joint. The microstructures were determined using an MET-2 optical microscope, a JEOL JIB-4501 scanning electron microscope equipped with an energy dispersive spectroscopy (EDS) spectrometer, model Bruker X/Flash 6/60, and an X spectrometer, and also equipped with an electron backscatter diffraction detector (EBSD). The obtained data was analyzed using HKL Channel 5 software. Results and discussion The base metal (Fig. 2) contains ferrite and granular carbides in the form of inclusions up to 1 μm in size uniformly distributed over the entire area of the section. It is known that part of the molybdenum is in ferrite, and chromium and carbon are in carbides; this fact provides the heat resistance of the steel [1–5]. It was observed that the base material consists mainly of α-ferrite grains with some Fe3C distributed around it. The heat-affected zone (HAZ) was the region where the elevated temperature was high enough (but below the melting point) to change the microstructure. During welding, the base material was transformed into smaller equiaxed grains in the HAZ due to the elevated temperature. New grains nucleated and grew at grain boundaries. However, the short duration of the welding process limited grain growth. In welded pipes without heat treatment in the weld joint zone at a distance of 7 mm in both directions, the main structure of the sorbite, there are separate areas of acicular bainite (Fig. 3). As expected, the grain size near the fusion line was significantly larger than at a distance. The peculiarity of bainitic ferrite was that the ferrite grew from the grain boundary of the preceding austenite to the inner grain and formed parallel laths. Bainitic ferrite arose as a result of a mixture of shear and diffusion transformations at high cooling rates. The hardness of the metal is 295–321 HB. Fig. 2. Base metal microstructure Fig. 3. Microstructure of welded pipe metal in the joint zone at a distance of 7 mm
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